Printing presses for printing on a web of paper or any other suitable printing medium often use an ink which must be heated after it is applied to the web to remove moisture and solvents from the ink. Typically, the web is heated to a temperature between approximately 280.degree. F. and approximately 375.degree. F. (138.degree.-191.degree. C.). Of course, the temperature to which the web is heated depends upon the type of ink used and the composition of the web, as will be evident to those of ordinary skill in the art. Once the moisture and solvents have been removed by this heating process, the ink, and thus the web, must be cooled in order for the ink to set. In addition, the printing press itself must be cooled to protect printing press components from the high temperatures generated to heat the printed web.
Referring to FIG. 1, in a prior-art cooling system 20 for cooling a printed web 22 that has been heated as described above, four hollow, cylindrical drums or chill rolls 24 convey the web 22 away from a printing press 26 in the direction indicated by the arrow 27. Obviously, any other desired number of chill rolls 24 may be used to convey the web 22. A closed-loop circulation system 28 interconnects the chill rolls 24 to form two parallel-connected pairs, as shown. Water is circulated through the closed-loop system 28 including interior spaces 30 inside the hollow chill rolls 24 at a constant rate by a constant-speed pump 32 in order to cool the web 22 and set the ink imprinted thereon.
The temperature of the chill rolls 24 is detected by a sensor 34, and when the temperature exceeds a predetermined threshold, a control 36 opens a valve 38. When the valve 38 opens, water escapes from the otherwise closed-loop system 28 and is replaced by new, cooler water from a water supply 40 to maintain the temperature of the chill rolls 24 at a desired level. When the sensed temperature decreases below the predetermined threshold, the valve 38 is closed, and the water in the closed-loop system 28 continues to be recirculated through the system 28 by the pump 32.
In the prior-art cooling system 20, the pump 32 operates continuously at a constant speed and therefore requires a large amount of electric power to cool the chill rolls 24. Moreover, the prior-art system 20 does not always make the most efficient use of cooling water. Still further, the system 20 cools the ink rapidly and often causes the ink to undergo thermal shock which can adversely affect the appearance (e.g., reducing glossiness) of the dried ink, which is undesirable.